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Geophysical Survey Report
M20 J10a, Ashford , Kent
for
Heritage Network
October 2010
J2780
Bryony Marsh BA Adam Cooper MSc
Document Title: Geophysical Survey Report
M20 J10a, Ashford, Kent
Client: Heritage Network
Stratascan Job No: 2780
Techniques: Detailed magnetic survey (gradiometry)
National Grid Ref: TR 040 410
Plate 1: Survey Area 1, viewed from west to east.
Field Team: Glenn Rose BA (Hons), Melanie Biggs BSc (Hons)
Project Manager: Simon Haddrell B.Eng (Hons) AMBCS PIFA
Report written by: Bryony Marsh BA
CAD illustration by: Bryony Marsh BA, Adam Cooper MSc
Checked by: Peter Barker C.Eng MICE MCIWEM MIFA Stratascan Ltd.
Vineyard House
Upper Hook Road
Upton upon Severn
WR8 0SA
Tel: 01684 592266
Fax: 01684 594142
Email: [email protected]
www.stratascan.co.uk
Geophysical Survey
M20 J10a, Ashford, Kent
Heritage Network Ltd October 2010
© Stratascan Ltd 2010 Page No. 1
Job ref: 2780
1 SUMMARY OF RESULTS............................................................................................3
2 INTRODUCTION ..........................................................................................................3
2.1 Background synopsis ..............................................................................................3
2.2 Site location ............................................................................................................3
2.3 Description of site ...................................................................................................3
2.4 Geology and soils....................................................................................................5
2.5 Site history and archaeological potential .................................................................5
2.6 Survey objectives ....................................................................................................5
2.7 Survey methods.......................................................................................................5
3 METHODOLOGY .........................................................................................................6
3.1 Date of fieldwork ....................................................................................................6
3.2 Grid locations..........................................................................................................6
3.3 Survey equipment ...................................................................................................6
3.4 Sampling interval, depth of scan, resolution and data capture ..................................7
3.4.1 Sampling interval ............................................................................................7
3.4.2 Depth of scan and resolution ...........................................................................7
3.4.3 Data capture ....................................................................................................7
3.5 Processing, presentation of results and interpretation...............................................7
3.5.1 Processing .......................................................................................................7
3.5.2 Presentation of results and interpretation .........................................................8
4 RESULTS ......................................................................................................................8
5 CONCLUSION ............................................................................................................10
APPENDIX A – Basic principles of magnetic survey .......................................................12
APPENDIX B – Glossary of magnetic anomalies .............................................................13
Geophysical Survey
M20 J10a, Ashford, Kent
Heritage Network Ltd October 2010
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LIST OF FIGURES
Figure 1 1:25 000 General location plan & site overview
Figure 2 1:1000 & 1:1250 Site plan showing location of survey grids and referencing
Figure 3 1:1000 & 1:1250 Plot of raw gradiometer data
Figure 4 1:1000 & 1:1250 Colour plot of raw gradiometer data showing extreme
magnetic values
Figure 5 1:1000 & 1:1250 Plot of processed gradiometer data
Figure 6 1:1000 & 1:1250 Abstraction and interpretation of gradiometer anomalies
Geophysical Survey
M20 J10a, Ashford, Kent
Heritage Network Ltd October 2010
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1 SUMMARY OF RESULTS
A detailed gradiometry survey was conducted over approximately 9.7 hectares of mixed
arable and scrubland in close proximity to the M20 Junction 10A, in Ashford, Kent. The
gradiometer data has identified positive linear and area anomalies which have been
interpreted as in-filled cut features of possible archaeological origin. Two negative
linear responses are identified which may indicate the presence of former banks or
earthworks. Discrete positive anomalies are seen across much of the site and may
indicate the presence of former pits which may be of archaeological origin. An area of
strong magnetic variation is observed which correlates with the location of a former
quarry identified through historic mapping. Numerous modern services and associated
magnetic disturbance are present on the site alongside a large area of made ground and
patches of magnetic debris. Numerous responses indicating the presence of ferrous
objects have been identified and magnetic disturbance associated with the field
boundaries can be seen.
2 INTRODUCTION
2.1 Background synopsis
Stratascan were commissioned to undertake a geophysical survey of an area outlined for
development as part of a road improvement scheme. This survey forms part of an
archaeological investigation being undertaken by Heritage Network Ltd.
2.2 Site location
The site is located to the east of Ashford, Kent at OS ref TR 040 410. The survey area is
split up into small parcels of land which are located in close proximity to the M20.
2.3 Description of site
The survey area is 9.7 hectares of land divided into seven parcels (The numbering
system used has been carried over from that put in place by Heritage Network Ltd):
Area 1 – Consisting of rough scrubland with obstructions caused by brambles and trees.
(See plates 2 & 3).
Plates 2 & 3: (Left) Area 1, looking north & (Right) Area 1, looking east.
Geophysical Survey
M20 J10a, Ashford, Kent
Heritage Network Ltd October 2010
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Area 2 & 3 - Currently in use as arable farmland. This area had been recently ploughed
at the time of the survey. (See plate 4).
Areas 5 & 6 – These are two small parcels of land situated directly to the south of the
M20. The areas have been left as scrubland and are bordered by trees. (See Plate 5).
Plates 4 & 5: (Left) Area 3 & (Right) Area 5.
Area 8 – This area is a long thin strip of land adjacent to the M20. The eastern section of
the area had been fenced off at the time of the survey and was in use as a compound,
making it unsuitable for geophysical survey. A further area of the site was sectioned off
using metal fencing as shown in Plate 6. The ground had been ploughed within the
fenced off areas, with the remainder of the site left as rough scrubland (See Plate 7).
Plates 6 & 7: (Left) Area 8, viewed from the east and showing the fenced off areas.
(Right) Area 8 viewed from the centre of the survey area looking to the west.
Area 11 – A further small parcel of land located directly to the north of the M20. The
groundcover consists of rough scrubland and is bordered by trees and fencing to the
south. (See plate 8).
Geophysical Survey
M20 J10a, Ashford, Kent
Heritage Network Ltd October 2010
© Stratascan Ltd 2010 Page No. 5
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Plate 8: Area 11, viewed from the west looking to the east.
2.4 Geology and soils
The underlying geology is Lower Greensand Group and Woburn Sand (British
Geological Survey South Sheet, Fifth Edition Solid, 2007). There is no drift geology
present. (British Geological Survey South Sheet, First Edition Quaternary, 1977).
The overlying soils are unlisted due to the urban location (Soil Survey of England and
Wales, 1983. Soils of England and Wales, Sheet 6 South East England).
2.5 Site history and archaeological potential
No specific details were available to Stratascan.
2.6 Survey objectives
The objective of the survey was to locate any features of possible archaeological
significance in order that they may be assessed prior to development.
2.7 Survey methods
Detailed magnetic survey (gradiometry) was used as an efficient and effective method
of locating archaeological anomalies. More information regarding this technique is
included in the Methodology section below.
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3 METHODOLOGY
3.1 Date of fieldwork
The fieldwork was carried out over 5 days from 4th
October 2010 to 8th
October 2010.
Weather conditions during the survey were mostly fine and dry.
3.2 Grid locations
The location of the survey grids has been plotted in Figure 2 together with the
referencing information. Grids were set out using a Leica Smart Rover RTK GPS.
An RTK GPS (Real-time Kinematic Global Positioning System) can locate a point on
the ground to a far greater accuracy than a standard GPS unit. A standard GPS suffers
from errors created by satellite orbit errors, clock errors and atmospheric interference,
resulting in an accuracy of 5m-10m. An RTK system uses a single base station receiver
and a number of mobile units. The base station re-broadcasts the phase of the carrier it
measured, and the mobile units compare their own phase measurements with those they
received from the base station. A SmartNet RTK GPS uses Ordnance Survey’s network
of over 100 fixed base stations to give an accuracy of around 0.01m.
3.3 Survey equipment and gradiometer configuration
Although the changes in the magnetic field resulting from differing features in the soil
are usually weak, changes as small as 0.2 nanoTesla (nT) in an overall field strength of
48,000nT, can be accurately detected using an appropriate instrument.
The mapping of the anomaly in a systematic manner will allow an estimate of the type
of material present beneath the surface. Strong magnetic anomalies will be generated by
buried iron-based objects or by kilns or hearths. More subtle anomalies such as pits and
ditches can be seen if they contain more humic material which is normally rich in
magnetic iron oxides when compared with the subsoil.
To illustrate this point, the cutting and subsequent silting or backfilling of a ditch may
result in a larger volume of weakly magnetic material being accumulated in the trench
compared to the undisturbed subsoil. A weak magnetic anomaly should therefore appear
in plan along the line of the ditch.
The magnetic survey was carried out using a dual sensor Grad601-2 Magnetic
Gradiometer manufactured by Bartington Instruments Ltd. The instrument consists of
two fluxgates very accurately aligned to nullify the effects of the Earth's magnetic field.
Readings relate to the difference in localised magnetic anomalies compared with the
general magnetic background. The Grad601-2 consists of two high stability fluxgate
gradiometers suspended on a single frame. Each gradiometer has a 1m separation
between the sensing elements so enhancing the response to weak anomalies.
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3.4 Sampling interval, depth of scan, resolution and data capture
3.4.1 Sampling interval
Readings were taken at 0.25m centres along traverses 1m apart. This equates to 3600
sampling points in a full 30m x 30m grid.
3.4.2 Depth of scan and resolution
The Grad 601 has a typical depth of penetration of 0.5m to 1.0m. This would be
increased if strongly magnetic objects have been buried in the site. The collection of
data at 0.25m centres provides an optimum methodology for the task balancing cost and
time with resolution.
3.4.3 Data capture
The readings are logged consecutively into the data logger which in turn is daily down-
loaded into a portable computer whilst on site. At the end of each job, data is transferred
to the office for processing and presentation.
3.5 Processing, presentation of results and interpretation
3.5.1 Processing
Processing is performed using specialist software known as Geoplot 3 alongside in-
house software. This can emphasise various aspects contained within the data but which
are often not easily seen in the raw data. Basic processing of the magnetic data involves
'flattening' the background levels with respect to adjacent traverses and adjacent grids.
'Despiking' is also performed to remove the anomalies resulting from small iron objects
often found on agricultural land. Once the basic processing has flattened the background
it is then possible to carry out further processing which may include low pass filtering to
reduce 'noise' in the data and hence emphasise the archaeological or man-made
anomalies.
The following schedule shows the basic processing carried out on all processed
gradiometer data used in this report:
1. Despike (useful for display and allows further processing functions
to be carried out more effectively by removing extreme
data values)
Geoplot parameters:
X radius = 1, y radius = 1, threshold = 3 std. dev.
Spike replacement = mean
Geophysical Survey
M20 J10a, Ashford, Kent
Heritage Network Ltd October 2010
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2. Zero mean traverse (sets the background mean of each traverse within a grid
to zero and is useful for removing striping effects)
Geoplot parameters:
Least mean square fit = off
3.5.2 Presentation of results and interpretation
The presentation of the data for each site involves a print-out of the raw data both as a
greyscale plot (Figure 3) and a colour plot showing extreme magnetic values (Figure 4),
together with a greyscale plot of the processed data (Figure 5). Magnetic anomalies
have been identified and plotted onto the 'Abstraction and Interpretation of Anomalies'
drawing for the site (Figure 6).
4 RESULTS
The gradiometer data collected at the M20 J10a site shows little evidence of anomalies
likely to be of archaeological origin, however much of the site displays evidence of
strong magnetic responses typical of modern activity.
Area 1
The survey data collected from Area 1 identifies three distinct linear anomalies of high
amplitude which have been interpreted as relating to modern services or field drains.
These anomalies have a halo of magnetic disturbance associated with them and further
disturbance can be observed relating to the western and northern field boundaries. To
the east of the largest linear response is a large area of strong magnetic debris which has
been interpreted as relating to made ground. Magnetic debris of a lesser amplitude can
be seen in the west and far east of the area. Numerous strong positive anomalies with
associated negative responses are seen throughout the area and are interpreted as ferrous
objects. Areas of magnetic disturbance relating to nearby metallic objects are also
present. It is possible that the high level of strong magnetic responses present in this
area is obscuring anomalies of a lesser amplitude – which would include responses
relating to archaeological activity. Two small weak positive area anomalies can be seen
to the far east of Area 1 and are of unknown origin.
Area 2
Numerous discrete positive anomalies have been identified in Area 2 which are
interpreted as possible pits and may be of archaeological origin. Areas of magnetic
disturbance are present to the south and east and are associated with the field
boundaries. Also seen is a single positive anomaly with associated response indicating a
ferrous object.
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M20 J10a, Ashford, Kent
Heritage Network Ltd October 2010
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Area 3
A positive linear response is identified in the south east of Area 3. This type of anomaly
is indicative of an in-filled cut feature such as a ditch and may be of archaeological
origin. Discrete positive anomalies, interpreted as possible pits can be seen throughout
the area. A small positive area anomaly has been identified close to the north eastern
field boundary and is interpreted as an in-filled cut feature of possible archaeological
origin. A further positive anomaly can be seen to the south of the area and appears to
have a weak negative response associated with it. Two similar responses of lesser
amplitude can also be seen to the south of the area. These anomalies may indicate
former cut and banked features and may be of archaeological interest.
Seen adjacent to the north eastern field boundary is an area of very strong magnetic
variation. This response correlates with the location of a former quarry which is
identified on historic mapping from 1898 to 1907. This strong response is interpreted as
relating to the backfilling of the quarry once it had gone out of use. A strong linear
anomaly again relating to a modern service cuts through the survey area and a band of
magnetic disturbance associated with it can be seen. Further disturbance relating to the
field boundaries is present along with magnetic debris and ferrous spikes.
Area 5 & 11
Two negative linear anomalies have been identified in Area 5 which are indicative of
former banks or earthworks. These responses are seen in close proximity to a number of
weak negative responses and positive area anomalies which may all be of
archaeological origin. Also seen within this area are numerous discrete anomalies or
possible pits.
A large area of strong magnetic disturbance is present to the east of both Area 5 and
Area 11 and a linear anomaly relating to it is present within Area 11. This anomaly
appears to be the continuation of the modern service identified in Area 3. Further
magnetic disturbance is present, associated with the field boundaries. A small area of
magnetic debris with a probable ferrous object within it can be seen to the north of Area
11.
Areas 6 & 8
Magnetic disturbance associated with the field boundaries has been identified in both
Areas 6 & 8. In Area 8, further disturbance is present relating to the metal fences
subdividing the area. A single anomaly interpreted as relating to a ferrous object is
present in Area 6 while moderate strength positive anomalies with associated negative
responses are seen in both areas. These responses are of unknown origin but may
indicate ferrous objects at depth. Small areas of magnetic debris are also present to the
centre and east of Area 8.
Geophysical Survey
M20 J10a, Ashford, Kent
Heritage Network Ltd October 2010
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5 CONCLUSION
The gradiometer data has identified a positive linear anomaly in Area 3 which is
interpreted as a possible in-filled cut feature such as a ditch. Also seen within this area
are three anomalies consisting of positive area anomalies surrounded by weak negative
halo’s which may also be of archaeological interest. Within Area 5, two negative linear
responses are seen which may indicate former banks which are seen in close proximity
to positive and weak negative area anomalies. Discrete positive responses interpreted as
possible pits are seen in Areas 2, 3 and 5.
Numerous modern services and associated magnetic disturbance appear within the data
along with areas of magnetic debris and made ground. An area of strong magnetic
variation seen in Area 3 correlates with the location of a former quarry present on
mapping from 1989 to 1907 and is likely to relate to the backfilling of the site. The
unfortunate presence of much magnetic disturbance caused by modern activity may be
obscuring weaker anomalies of archaeological origin.
Geophysical Survey
M20 J10a, Ashford, Kent
Heritage Network Ltd October 2010
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6 REFERENCES
British Geological Survey, 2007. Geological Survey Ten Mile Map, South Sheet, Fifth
Edition (Solid). British Geological Society.
Soil Survey of England and Wales, 1983. Soils of England and Wales, Sheet 6 South
East England.
British Geological Survey South Sheet, 1977. Geological Survey Ten Mile Map, South
Sheet First Edition (Quaternary). Institute of Geological Sciences.
Old Maps. 2010. www.old-maps.co.uk
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Heritage Network Ltd October 2010
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APPENDIX A – Basic principles of magnetic survey
Detailed magnetic survey can be used to effectively define areas of past human activity
by mapping spatial variation and contrast in the magnetic properties of soil, subsoil and
bedrock.
Weakly magnetic iron minerals are always present within the soil and areas of
enhancement relate to increases in magnetic susceptibility and permanently magnetised
thermoremanent material.
Magnetic susceptibility relates to the induced magnetism of a material when in the
presence of a magnetic field. This magnetism can be considered as effectively
permanent as it exists within the Earth’s magnetic field. Magnetic susceptibility can
become enhanced due to burning and complex biological or fermentation processes.
Thermoremanence is a permanent magnetism acquired by iron minerals that, after
heating to a specific temperature known as the Curie Point, are effectively demagnetised
followed by re-magnetisation by the Earth’s magnetic field on cooling.
Thermoremanent archaeological features can include hearths and kilns and material
such as brick and tile may be magnetised through the same process.
Silting and deliberate infilling of ditches and pits with magnetically enhanced soil
creates a relative contrast against the much lower levels of magnetism within the subsoil
into which the feature is cut. Systematic mapping of magnetic anomalies will produce
linear and discrete areas of enhancement allowing assessment and characterisation of
subsurface features. Material such as subsoil and non-magnetic bedrock used to create
former earthworks and walls may be mapped as areas of lower enhancement compared
to surrounding soils.
Magnetic survey is carried out using a fluxgate gradiometer which is a passive
instrument consisting of two sensors mounted vertically either 0.5 or 1m apart. The
instrument is carried about 30cm above the ground surface and the top sensor measures
the Earth’s magnetic field whilst the lower sensor measures the same field but is also
more affected by any localised buried field. The difference between the two sensors will
relate to the strength of a magnetic field created by a buried feature, if no field is present
the difference will be close to zero as the magnetic field measured by both sensors will
be the same.
Factors affecting the magnetic survey may include soil type, local geology, previous
human activity, disturbance from modern services etc.
Geophysical Survey
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Heritage Network Ltd October 2010
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APPENDIX B – Glossary of magnetic anomalies
Bipolar
A bipolar anomaly is one that is composed of both a positive
response and a negative response. It can be made up of any number
of positive responses and negative responses. For example a pipeline
consisting of alternating positive and negative anomalies is said to
be bipolar. See also dipolar which has only one area of each polarity.
The interpretation of the anomaly will depend on the magnitude of
the magnetic field strength. A weak response may be caused by a
clay field drain while a strong response will probably be caused by a
metallic service.
Dipolar
This consists of a single positive anomaly with an associated
negative response. There should be no separation between the two
polarities of response. These responses will be created by a single
feature. The interpretation of the anomaly will depend on the
magnitude of the magnetic measurements. A very strong anomaly is
likely to be caused by a ferrous object.
Positive anomaly with associated negative response
See bipolar and dipolar.
Positive linear
A linear response which is entirely positive in polarity. These are
usually related to in-filled cut features where the fill material is
magnetically enhanced compared to the surrounding matrix. They
can be caused by ditches of an archaeological origin, but also former
field boundaries, ploughing activity and some may even have a
natural origin.
Geophysical Survey
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Heritage Network Ltd October 2010
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Positive linear anomaly with associated negative response
A positive linear anomaly which has a negative anomaly located
adjacently. This will be caused by a single feature. In the example
shown this is likely to be a single length of wire/cable probably
relating to a modern service. Magnetically weaker responses may
relate to earthwork style features and field boundaries.
Positive point/area
These are generally spatially small responses, perhaps covering just
3 or 4 reading nodes. They are entirely positive in polarity. Similar
to positive linear anomalies they are generally caused by in-filled cut
features. These include pits of an archaeological origin, possible tree
bowls or other naturally occurring depressions in the ground.
Magnetic debris
Magnetic debris consists of numerous dipolar responses spread over
an area. If the amplitude of response is low (+/-3nT) then the origin
is likely to represent general ground disturbance with no clear cause,
it may be related to something as simple as an area of dug or mixed
earth. A stronger anomaly (+/-250nT) is more indicative of a spread
of ferrous debris. Moderately strong anomalies may be the result of
a spread of thermoremanent material such as bricks or ash.
Magnetic disturbance
Magnetic disturbance is high amplitude and can be composed of
either a bipolar anomaly, or a single polarity response. It is
essentially associated with magnetic interference from modern
ferrous structures such as fencing, vehicles or buildings, and as a
result is commonly found around the perimeter of a site near to
boundary fences.
Geophysical Survey
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Heritage Network Ltd October 2010
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Negative linear
A linear response which is entirely negative in polarity. These are
generally caused by earthen banks where material with a lower
magnetic magnitude relative the background top soil is built up. See
also ploughing activity.
Negative point/area Opposite to positive point anomalies these responses may be caused by raised areas or earthen
banks. These could be of an archaeological origin or may have a natural origin.
Ploughing activity
Ploughing activity can often be visualised by a series of parallel
linear anomalies. These can be of either positive polarity or negative
polarity depending on site specifics. It can be difficult to distinguish
between ancient ploughing and more modern ploughing, clues such
as the separation of each linear, straightness, strength of response
and cross cutting relationships can be used to aid this, although none
of these can be guaranteed to differentiate between different phases
of activity.
Polarity
Term used to describe the measurement of the magnetic response. An anomaly can have a
positive polarity (values above 0nT) and/or a negative polarity (values below 0nT).
Strength of response
The amplitude of a magnetic response is an important factor in assigning an interpretation to a
particular anomaly. For example a positive anomaly covering a 10m2 area may have values up
to around 3000nT, in which case it is likely to be caused by modern magnetic interference.
However, the same size and shaped anomaly but with values up to only 4nT may have a
natural origin. Trace plots are used to show the amplitude of response.
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Thermoremanent response
A feature which has been subject to heat may result in it acquiring a magnetic field. This can
be anything up to approximately +/-100 nT in value. These features include clay fired drains,
brick, bonfires, kilns, hearths and even pottery. If the heat application has occurred in situ
(e.g. a kiln) then the response is likely to be bipolar compared to if the heated objects have
been disturbed and moved relative to each other, in which case they are more likely to take an
irregular form and may display a debris style response (e.g. ash).
Weak background variations
Weakly magnetic wide scale variations within the data can
sometimes be seen within sites. These usually have no specific
structure but can often appear curvy and sinuous in form. They are
likely to be the result of natural features, such as soil creep, dried up
(or seasonal) streams. They can also be caused by changes in the
underlying geology or soil type which may contain unpredictable
distributions of magnetic minerals, and are usually apparent in
several locations across a site.